The present invention relates generally to photonic band gap devices and methods of manufacturing same, and more particularly to a photonic band gap structure suitable for filtering in the optical wavelength region and a method of manufacturing same.
Photonic band gap (PBG) structures are periodic dielectric or metallic structures that exhibit frequency regions in which electromagnetic waves cannot propagate. The interest in PBGs arises from the fact that photon behavior in a dielectric structure is similar to the behavior of electrons in a semiconductor. The periodic arrangement of atoms in a semiconductor lattice opens up forbidden gaps in the energy band diagram for the electrons. Similarly in all-dielectric PBG structures, the periodic placement of dielectric xe2x80x9catomsxe2x80x9d opens up forbidden gaps in the photon energy bands.
The idea of PBGs has led to the proposal of many novel applications at optical wavelengths, such as thresholdless lasers, single-mode light-emitting-diodes and optical wave guides. In addition, PBGs are already being used in the millimeter and microwave regimes, where the applications include efficient reflectors, antennas, filters, sources and wave guides. They have also found possible applications as infrared filters.
The PBG structures behave as ideal reflectors in the band gap region. Depending on the directional periodicity of these dielectric structures, the band gap may exist in 1-D, 2-D or all the three directions. Various lattice geometries were studied to find a periodic structure that would exhibit a photonic band gap in all the directions. After several unsuccessful attempts in finding the right lattice geometry using xe2x80x9ctrial-and-errorxe2x80x9d techniques, researchers at Iowa State University were first to predict the existence of a complete band gap in a periodic dielectric structure arranged in diamond lattice geometry. Diamond lattice ran structures were calculated to have large gaps for a refractive index ratio between the two at dielectrics as low as two.
Further research indicated that, similar to the impurity doping in a semiconductor, localized electromagnetic modes can be created in the band gap region of PBG structures by introducing defects that disturb the periodicity of the structure. This defect can be achieved by removing a part of the PBG structure, thus creating states similar to the semiconductor behavior with acceptor atoms. The defect can also be achieved by adding extra material to the crystal, which acts like a donor atom of a semiconductor. The defect gives rise to donor modes which have their origin at the bottom of the conduction band. Experiments have shown that the acceptor modes, acting like cavities, are of greater importance with their highly localized and single-mode cavity characteristics. In photonic crystals with defects, the transmission spectrum is changed by the presence of a narrow transmission peak within the band gap. Defect peaks with quality factors in the range of 1000-2000 have been experimentally demonstrated.
After the initial research into the existence of photonic band gap, there was an increased effort to find new structures that could be more easily fabricated. These fabrication techniques include creating the PBG structure through emulsions, with carbon structures, and by creating crystals by a liquid-phase chemical reaction to infiltrate a polystyrene template. These manufacturing techniques allow the defect to be introduced by adding extra material to the crystal or by removing material after the photonic band gap structure has been built. This increases the manufacturing time and cost of the photonic band gap structures. What is needed is a way to manufacture a photonic band gap structure that provides the capability to introduce defects without adding extra material or removing material after the photonic band gap structure is built.
It is an object of the instant invention to overcome at least some of the aforementioned and other known problems existing in the art. More particularly, it is an object of the instant invention to provide a new and improved method of manufacturing a photonic band gap structure allowing operation in the optical region. It is a further object of the instant invention to provide a new and improved method of manufacturing PBGs having a three-dimensional photonic band gap structure. It is also an object of the instant invention to provide a new and improved method of manufacturing photonic band gap structures having adjustable process steps resulting in varying photonic band gap structures whose performance characteristics are adjusted to meet specific performance requirements. Furthermore, it is an object of the instant invention to provide a new and improved method of manufacturing photonic band gap structures utilizing micro-transfer molded structures.
In view of the above objects, it is a feature of the instant invention to provide a method of manufacturing photonic band gap structures which utilize simple cost effective micro-transfer based construction techniques. It is an additional feature of the instant invention that the photonic band gap structures resulting from the method of manufacturing of the instant invention have characteristics of simple high pass filters, band stop filters, or filters having more complex transmission characteristics in the optical region depending upon the periodic pattern of the grids. Furthermore, it is a feature of the instant invention that the photonic band gap structures resulting from the method of the instant invention are lightweight and compact.
In accordance with an embodiment of the instant invention, a method of manufacturing a photonic band gap structure operable in the optical region comprises the steps of: a) creating a patterned template for an elastomeric mold; b) fabricating an elastomeric mold from poly-dimethylsiloxane (PDMS) or other suitable polymer; c) filling the elastomeric mold with a second polymer; d) stamping the second polymer (epoxy) by making contact with a substrate or multilayer structure; e) introducing a ceramic bearing material (e.g., a sol or slurry) into the second polymer to form a ceramic and epoxy structure; f) heating the ceramic-epoxy structure to remove the epoxy.
The method of the instant invention may be repeated to produce multi-layer structures of 1, 2, 3, 4, 5, etc. layers. These method steps may be performed so that the thickness of dielectric layers separating the patterns are approximately equal, or are unequal. These steps include, in the step of forming a second or middle pattern in a three layer structure, the step of introducing a defect in the pattern. This defect may be adjusted to control a parameter of a filter characteristic. The second polymer may be made with a slurry or sol gel infiltration and may also be made with metal infiltration. The second polymer may also be an epoxy or other suitable polymers of appropriate viscosity and with physical and chemical properties that allow the building of a layered structure and removal via pyrolysis. Additionally, each step of forming a pattern may include the step of introducing a defect in the pattern.
In one embodiment, the photonic band gap structure is formed with layers of rods stacked on top of each other, each layer having its axes oriented at 90xc2x0 with respect to adjacent layers, alternate layers having their axes parallel to each other with the rods of one layer in offset between the rods of the other layer forming a three-dimensional structure of stacked layers having a four-layer periodicity, the dielectric rods arranged with parallel axes at a given spacing to form a planar layer and arranged in a material having a different and contrasting refractive index, the dimensions of the rods, the spacing between the rods and the refractive contrast of the materials selected to produce a photonic band gap operable in the optical region made by the method of the instant invention.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.